Convergence rate of some semi-groups to their invariant probability

Let us consider the following stochastic differential equation:(E)Xt=x+Bt−12∫0tb(Xs) ds,where (Bt)t⩾0 is a d-dimensional brownian motion starting at 0 and b a function from Rd to Rd which is a gradient field. We aim at studying the convergence rate of the semi-group associated to (E) to its invariant probability.